SAMARRA, Iraq – Insurgents posing as police destroyed the golden dome of one of Iraq’s holiest Shiite shrines Wednesday, setting off an unprecedented spasm of sectarian violence. Angry crowds thronged the streets, militiamen attacked Sunni mosques, and at least 19 people were killed.

With the gleaming dome of the 1,200-year-old Askariya shrine reduced to rubble, some Shiites lashed out at the United States as partly to blame.

The violence – many of the 90 attacks on Sunni mosques were carried out by Shiite militias – seemed to push Iraq closer to all-out civil war than at any point in the three years since the U.S.-led overthrow of Saddam Hussein.

Many leaders called for calm. “We are facing a major conspiracy that is targeting Iraq’s unity,” said President Jalal Talabani, a Kurd. “We should all stand hand in hand to prevent the danger of a civil war.”

President Bush pledged American help to restore the mosque after the bombing north of Baghdad, which dealt a severe blow to U.S. efforts to keep Iraq from falling deeper into sectarian violence.

“The terrorists in Iraq have again proven that they are enemies of all faiths and of all humanity,” Bush said. “The world must stand united against them, and steadfast behind the people of Iraq.”

U.S. Ambassador Zalmay Khalilzad and the top American commander in Iraq, Gen. George Casey, called the attack a deliberate attempt to foment sectarian strife and warned it was a “critical moment for Iraq.”

No one was reported injured in the bombing of the shrine in Samarra.

But at least 19 people, including three Sunni clerics, were killed in the reprisal attacks that followed, mainly in Baghdad and predominantly Shiite provinces to the south, according to the Iraqi Islamic Party, the country’s largest Sunni political group.

Many of the attacks appeared to have been carried out by Shiite militias that the United States wants to see disbanded.

In predominantly Shiite Basra, police said militiamen broke into a prison, hauled out 12 inmates, including two Egyptians, two Tunisians, a Libyan, a Saudi and a Turk, and shot them dead in reprisal for the shrine attack.

Major Sunni groups joined in condemning the attack, and a leading Sunni politician, Tariq al-Hashimi, urged clerics and politicians to calm the situation “before it spins out of control.”

The country’s top Shiite cleric, Grand Ayatollah Ali al-Sistani, sent instructions to his followers forbidding attacks on Sunni mosques, and called for seven days of mourning.

But he hinted, as did Vice President Adil Abdul-Mahdi, that religious militias could be given a bigger security role if the government cannot protecting holy shrines – an ominous sign of the Shiite reaction ahead.

Both Sunnis and the United States fear the rise of such militias, which the disaffected minority views as little more than death squads. American commanders believe they undercut efforts to create a professional Iraqi army and police force – a key step toward the eventual drawdown of U.S. forces.

The new tensions came as Iraq’s various factions have been struggling to assemble a government after the Dec. 15 parliamentary elections.

The Shiite fury sparked by Wednesday’s bombings – the third major attack against Shiite targets in as many days – raised the likelihood that Shiite religious parties will reject U.S. demands to curb militias.

The Askariya shrine, also known as the Golden Mosque, contains the tombs of two revered Shiite imams, who are considered by Shiites to be among the successors of the Prophet Muhammad.

No group claimed responsibility for the 6:55 a.m. assault on the shrine in Samarra, a mostly Sunni Arab city 60 miles north of Baghdad, carried out by four insurgents disguised as police. But suspicion fell on Sunni extremist groups.

Thousands of demonstrators crowded near the wrecked shrine, and Iraqis picked through the debris, pulling out artifacts and copies of the Muslim holy book, the Quran, which they waved, along with Iraqi flags.

U.S. and Iraqi forces surrounded the Samarra shrine and searched nearby houses. About 500 soldiers were sent to Sunni neighborhoods in Baghdad to prevent clashes.

National Security Adviser Mouwafak al-Rubaie said 10 people were detained for questioning about the bombing. The Interior Ministry put the number at nine and said they included five guards.

In the hours after the attack, more than 90 Sunni mosques were attacked with automatic rifles and rocket-propelled grenades, burned or taken over by Shiites, the Iraqi Islamic Party said.

Large protests erupted in Shiite parts of Baghdad and in cities throughout the Shiite heartland to the south. In Basra, Shiite militants traded rifle and rocket-propelled grenade fire with guards at the office of the Iraqi Islamic Party. Smoke billowed from the building.

Shiite protesters later set fire to a Sunni shrine containing the seventh century tomb of Talha bin Obeid-Allah, a companion of Muhammad, on the outskirts of Basra.

“This criminal act aims at igniting civil strife,” said Mahmoud al-Samarie, a 28-year-old builder. “We demand an investigation so that the criminals who did this be punished. If the government fails to do so, then we will take up arms and chase the people behind this attack.”